What will alien life be like? What will it look like? Will
there be creatures on other worlds similar to ourselves, or other kinds
of life on Earth? Or will extraterrestrial life sometimes be very different
from anything we know about? Scientists, science fiction writers, artists,
and others have, for many years, speculated about the appearance and nature
of life forms elsewhere in the universe. Here are just a few of the ideas
they've come up with.

Aliens like us

Call it a failure of the imagination or an easy way to save on props and
make-up, but many writers and film producers have opted to depict smart
aliens as being very much like ourselves. Early writers, from ancient to
Victorian times, made little attempt to invent aliens that weren't either
based on humans or some pick-and-mix blend of creatures that we're familiar
with on Earth. More recently, film and TV producers have favored humanoid
aliens for obvious reasons, although the advent of CGI has allowed them
greater creative freedom. See also anthropomorphism.

Mr Spock's pointed ears, upward-pointing eyebrows,
and green blood are the only concessions
to his unearthly nature. Yet at the time of his first appearance in
Star Trek, the notion of an alien
as the central character in a drama series was considered highly controversial
and TV executives wanted to do away with the prosthetic ears!

Bipedal – but on the weird side

One of the reasons we have an advanced technology is that
we stand on two legs (see bipedalism).
When our ancestors abandoned a life on all fours, they freed up their front
appendages to become dextrous hands that could finely manipulate things
and make tools. Dolphins are smart, but
they lack anything equivalent to hands so are essentially non-technological.

Aliens are often imagined to be bipedal even if they are unlike us in other
ways.

The alien from the Alien
series of movies. Ruthlessly predatory in nature, all-round thoroughly
unpleasant, and, curiously, blind, it nevertheless is humanoid in
overall body-plan, conveniently allowing it to be played by an actor.

A "gray": in the annals of ufology, a purported advanced
extraterrestrial commonly implicated in alien visitation.

Aliens from Earth

Our own planet has plenty of alien-looking fauna and flora
– enough to hint at the astonishing variety of forms that probably
exists on other worlds.

Enough to bring a smile to anyone's face: the Blobfish
(Genus Psychrolutes)

Another denizen of the deep ocean: an annelid worm
found near an undersea vent.

Outer limits

Because at present we know of only one instance of life –
terrestrial life – we have no idea how broad the concept of life can
be. The discovery of extremophiles
has increased optimism that life can adapt to a great range of environments.
And the discovery of large numbers of extrasolar worlds suggests there may
be plenty of places where biology can take hold throughout the Galaxy and
beyond.

Life based on silicon? The Horta, from the original
Star Trek series, is an example of silicon-based life in science fiction.
But is a radically alternative biochemistry, based on an element other
than carbon, feasible? Many scientists would argue that carbon is
unique in its ability to form complex molecules which are both stable
and yet susceptible to chemical change; only practical astrobiology
will put this notion to the test.

The remote possibility exists that some life might
originate, not on planets or moons, but in interstellar space. Fred
Hoyle's Black Cloud is an SF example of such an organism. Shown here
is a diffuse nebula in the Milky Way (credit and copyright Steve Mandell
of Galaxy Images.)